Investigations on the Passivity of Iron in Borate and Phosphate Buffers, pH 8.4

I.V. Kobanenko, P.Schmuki

In the present work surface analytical experiments (XPS and AES) on the passive film on iron have been carried out, in order to refine or verify models previously suggested for reaction mechanisms of passive films on iron [1-5]. Of a special interest is to elucidate the influence of solution chemistry (especially a comparison of borate and phosphate buffer, pH 8.4) on the nature of the passive film on Fe, and the mechanism of the reduction of the passive film.

The nature of the passive film (chemical composition and thickness) depends on the buffer solution used for passivation, even at an identical pH value. In the passive film formed in phosphate buffer, pH 8.4, a significant amount of phosphates is found in the outer part of the film. Boron species, however, are not significantly incorporated in the passive film formed in borate buffer (Fig.1).

Fig.1. AES depth profiles for passive film formed on iron at +0.800V (SCE), a) in phosphate buffer;
b) in borate buffer

The mechanism of the reduction of the passive film depends strongly on the electrolyte composition. In both solutions, the current/time curves (Fig.2) show characteristics peaks during the reduction time. In borate buffer, cathodic polarization leads to reductive dissolu-tion of the passive film whereas in phosphate buffer the passive film is converted into me-tallic iron without dissolution. This conversion of the passive film into metallic iron pro-ceeds via a formation of an intermediate Fe (II) phosphate layer. The function of the phos-phates therefore is to capture the iron (II) species formed in the reduction reaction (very low solubility of Fe (II) phosphates) and therefore to hinder reductive dissolution.

Fig.2 Current/time-behaviour of Fe during a potentiostatic step from 0.8V (SCE) to –1.2V (SCE): a) in phos-phate buffer, b) in borate buffer.

The conversion of the passive film into metallic iron via the intermediate phosphate film seems to proceed laterally inhomogeneously over the sample surface.

Fig.3. Fe polarized at –1.2V (SCE) in phosphate buffer (after passivation at +0.800V (SCE)) removed from electrolyte at point 2 in fig.2a: a) SEM image of the surface, b) AES depth profile at location 1; c) AES depth profile at location 2.

This suggests that the reaction starts at certain active sites of the surface, and then the re-action front spreads over the sample surface. In order to clarify the role of phosphate spe-cies in the reduction process, the following experiments were carried out: Fe was polarized anodically in phosphate (borate) buffer and after that cathodically reduced in borate (phos-phate buffer) buffer. Similar current/time curves for this treatment and SEM images of samples were found. The current shows the same characteristics peaks during reduction, which are the same as in the case if the whole treatment was carried out in phosphate buffer. The SEM image (Fig. 3 a) shows the presence of different regions on the samples, which correspond to a native passive film (2 nm) and to a phosphate layer (5-7 nm). Thus the cathodic reduction mechanism of the passive film on iron is determined by the pres-ence/absence of phosphate species in the system. In the presence of the phosphate species, cathodic reduction of the passive film always proceeds through the intermediate stage of phosphate compound formation. The phosphate compound required for this type of reduc-tion mechanism can be formed either during anodic polarization or during cathodic reduc-tion.

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